1. Field of the Invention
The invention relates to liquid crystalline chiral nematic polycarbonates.
2. Description of the Background
Heating of shape-anisotropic substances may result in liquid crystalline phases, called mesophases. The individual phases differ by the spatial arrangement of the molecular centers on the one hand and by the molecular arrangement with respect to the long axes on the other hand (G. W. Gray, P. A. Winsor, Liquid Crystals and Plastic Crystals, Ellis Horwood Limited, Chichester 1974). The nematic liquid crystalline phase is distinguished by parallel orientation of the long axes of the molecule (one-dimensional order state). Provided that the molecules forming the nematic phase are chiral, what results is a chiral nematic (cholesteric) phase in which the long axes of the molecules form a helical superstructure perpendicular thereto (H. Baessler, Festkorperprobleme XI, 1971). The chiral moiety may either be present in the liquid crystalline molecule itself or be added as doping substance to the nematic phase, in which case the chiral nematic phase is induced. This phenomenon was investigated first on cholesterol derivatives (for example H. Baessler, M. M. Labes, J. Chem. Phys. 52 (1970) 631)).
The chiral nematic phase has special optical properties: a high optical rotation and a pronounced circular dichroism which arises owing to selective reflection of circularly polarized light within the chiral nematic layer. The different colors which are apparent depending on the angle of view depend on the pitch of the helical superstructure which in turn depends on the twisting ability of the chiral component. It is moreover possible to alter the pitch, and thus the wavelength range of the selectively reflected light, of a chiral nematic layer in particular by changing the concentration of a chiral doping substance. Chiral nematic systems of this type have interesting possibilities for practical application. Thus, it is possible by incorporating chiral moieties into mesogenic acrylic esters and orienting in the chiral nematic phase, eg. after photopolymerization, to prepare a stable colored network, although it is not then possible to change its concentration of chiral component (G. Galli, M. Laus, A. Angelon, Makromol. Chemie 187 (1986) 2289). It is possible by admixing noncrosslinkable chiral compounds with nematic acrylic esters and by photopolymerization to prepare a colored polymer which still contains large amounts of soluble components (I. Heyndricks, D. J. Broer, Mol. Cryst. Liq. Cryst. 203 (1991) 113). It is furthermore possible by random hydrosilylation of mixtures of cholesterol derivatives and acrylate-containing mesogens with defined cyclic siloxanes and subsequent photopolymerization to obtain a chiral nematic network in which the chiral component may comprise up to 50% of the material employed; however, these polymers still contain marked amounts of soluble materials (F. H. Kreuzer, R. Mauerer, Ch. Muller-Rees, J. Stohrer, Presentation No. 7, 22nd meeting on liquid crystals, Freiburg, 1993).
DE-A-35 35 547 describes a process in which a mixture of cholesterol-containing monoacrylates can be converted by photopolymerization into chiral nematic layers. However, the total content of the chiral component in the mixture is about 94%. Although the mechanical stability of such a material as pure side-chain polymer is not very great, the stability can be increased only by highly crosslinking diluents.
Numerous chiral nematic polyesters in which the mesogenic structures are incorporated into the main chain are also generally known, eg. from S. Vilasagar, A. Blumstein, Mol. Cryst. Liq. Cryst. (1980), 56 (8), 263-9; A. Blumstein, S. Vilasagar, S. Ponratham, S. B. Clough, R. B. Blumstein, G. Maret, J. Polym. Sci., Polym. Phys. Ed. (1982), 20 (5), 877-92; E. Chiellini, G. Galli, C. Malanga, N. Spassky, Polym. Bull. (1983), 9 (6-7), 336-43; H. J. Park, J. I. Jin, R. W. Leng, Polymer (1985), 26 (9), 1301-6; J. I. Jin, E. J. Choi, K. Y. Lee, Polym. J. (1986), 18 (1), 99.101; J. I. Jin, S. C. Lee, S. D. Chi, J. H. Chang; Pollimo (1986), 10 (4), 382-8; J. I. Jin, E. J. Choi, B. W. Jo, Pollimo (1986), 10 (6), 635-40; J. M. G. Cowie, E. H. Wu, Makromol. Chem. (1988), 189 (7), 1511-16; V. V. Zuev, I. G. Denisov, S. S. Skorokhodov, Vysokomol. Soedin., Ser. A (1989, 31 (5), 1056-61; A. S. Angeloni, D. Caretti, C. Carlini, E. Chiellini, G. Galli, A. Altomare, R. Solaro, M. Laus, Liq. Cryst. (1986), 4 (5), 513-27; K. Fujishiro, R. W. Lenz, Macromolecules (1992), 25 (1), 88-95; K. Fujishiro, R. W. Lenz, Macromolecules (1992), 25 (1), 81-7; V. V. Zuev, I. G. Denisov, S. S. Skorokhodov, Vysokomol. Soedin., Ser. B (1992), 34 (3), 47-54; V. V. Zuev, I. G. Denisov, S. S. Skorokhodov Vysokomol. Soedin., Ser. B (1989), 31 (2), 130-2.
These polyesters usually show narrow ranges of existence of the chiral nematic phase and contain predominantly open-chain chiral components which have little twisting ability so that relatively large contents of these components are necessary in order to obtain a color. This means that the choice of the remaining polyester constituents is restricted, for example with respect to their mechanical properties.
DE-A-19504913.6 describes chiral nematic polyesters with chiral diol components which have a strong twisting effect, in particular dianhydride saccharides, and with wide liquid crystalline phase ranges.
EP-A-682 092 describes surface coatings based on chiral nematic polymers. The examples mentioned are exclusively polyesters prepared by polycondensation of dicarboxylic acids and diols. Polycarbonates are not mentioned.
Chiral nematic polycarbonates have not hitherto been disclosed. The carbonate group is, because of its nonlinear structure, less suitable than the ester group for stabilizing a liquid crystalline phase. Nevertheless, it has been possible to prepare aromatic polycarbonates which contain para-linked diphenols, in particular 4,4'-dihydroxybiphenyl (DHB), and are able to form a nematic phase over a wide temperature range (Kricheldorf, H. R.; Lubbers, D., Makromol. Rapid Communc. 1989, 10, 383; Kricheldorf, H. R.; Lubbers, D., Macromolecules 1990, 23, 2656; Sun, S.-J.; Chang, TzCh., J. Polym. Sci., Part A, Polym. Chem. 1993, 31, 2237). However, there are problems in using chiral comonomers because they destabilize the liquid crystalline phase of polycarbonates more strongly than do their nonchiral analogs.